Cold start, long cranking time '91 500sl
 

I have a '91 500 sl that at times will crank for 5 seconds before starting then stumble a bit but run fine after. Once it gets running it is fine for the rest of the day. I have limited access to manuals for this car which seems to be a common problem but have no doubt it is related to the cold start valve or temp sensor etc...any help would be greatly appreciated.